1. Field of the Invention
The present invention relates to a reference voltage circuit, and more particularly, to a reference voltage circuit having a soft start function in which a reference voltage gradually rises after an elapse of a predetermined time period.
2. Description of the Related Art
Generally, in a reference voltage circuit having a soft start function, a charge period of charging a capacitor by a constant current source is set as a soft start time period. After a charged voltage exceeds a predetermined voltage, a switch is switched, to thereby transit from the soft start voltage to a predetermined reference voltage (for example, see Japanese Patent Application Laid-open No. 2000-56843).
A conventional reference voltage circuit is described. FIG. 2 is a circuit diagram of the conventional reference voltage circuit. The reference voltage circuit includes a constant voltage source 101 and a soft start circuit. The soft start circuit includes a comparator 103, a delay circuit 104, a constant current source 102, a capacitor C, a resistor R, and switches SW1 to SW3.
A connection point between the constant current source 102 and the capacitor C is connected to an output terminal Vref of the reference voltage circuit. The comparator 103 has a non-inverting input terminal connected to the output terminal Vref, and an inverting input terminal connected to an output terminal of the constant voltage source 101 via an offset voltage Vos. Further, the comparator 103 has an output terminal connected to the switch SW2, the constant current source 102, and the delay circuit 104. The delay circuit 104 has an output terminal connected to the switch SW3.
The capacitor C receives a constant current Ic from the constant current source 102 to be charged. The comparator 103 compares a voltage obtained by subtracting the predetermined offset voltage Vos from an output voltage Vbgr of the constant voltage source 101 with a voltage at the connection point between the constant current source 102 and the capacitor C. Then, the comparator 103 outputs an output voltage reflecting the comparison result. When the voltage at the connection point between the constant current source 102 and the capacitor C is higher than the voltage obtained by subtracting the predetermined offset voltage Vos from the output voltage Vbgr of the constant voltage source 101, the switch SW2 is turned ON, the constant current source 102 stops supplying current, and the delay circuit 104 starts its operation. When the switch SW2 is turned ON, the capacitor C is charged by the constant voltage source 101 via the resistor R with an RC time constant. The output of the delay circuit 104 is connected to the switch SW3, and causes the switch SW3 to turn ON after an elapse of a predetermined time period from the operation start of the delay circuit 104. When the switch SW3 is turned ON, the output voltage Vbgr of the constant voltage source 101 is directly connected to the reference voltage Vref.
An operation of the conventional reference voltage circuit is described.
Under a state in which the switch SW1 is ON, the operation of the reference voltage circuit is stopped, and the reference voltage of the output terminal Vref is 0 V.
When the switch SW1 is turned OFF, the reference voltage circuit starts its operation. The capacitor C receives the constant current Ic from the constant current source 102 to start constant current charging. At this time, the reference voltage Vref increases linearly depending on the constant current Ic and the capacity of the capacitor C. When the voltage charged in the capacitor C exceeds Vbgr-Vos, the output signal of the comparator 103 is inverted. Thus, the switch SW2 is turned ON, the constant current source 102 stops supplying current, and the delay circuit 104 starts its operation. Because the constant current source 102 stops supplying current, the capacitor C is charged with power supplied from the output voltage Vbgr of the constant voltage source 101 via the resistor R.
After an elapse of a predetermined time period from the operation start of the delay circuit 104, the switch SW3 is turned ON, and thus the output voltage Vbgr of the constant voltage source 101 is directly provided as the reference voltage Vref.
In the conventional reference voltage circuit, switches are switched to set a soft start period and a predetermined reference voltage Vref. In this case, in order to generate switching signals for the switches, it is necessary to provide a comparator for comparing the internal reference voltage with the soft start voltage, and a delay circuit. As a result, the circuit size increases.
Further, there is a problem in that there appears discontinuity in the linearly increasing reference voltage because switching between the soft start period and the reference voltage output period is performed by the switches.